This invention relates to manufacturing graphite fibers by a process that comprises thermally decomposing a methane gas adjacent a suitable iron chromium alloy, such as stainless steel. More particularly, this invention relates to growing graphite fibers from the methane decomposition products on a surface that is pretreated to produce a high yield of the fibers.
U.S. Pat. No. 4,391,787 issued July 5, 1983, and assigned to the present assignee, describes a process for manufacturing thin, straight graphite fibers suitable for filler in plastic composites. In a preferred embodiment, a natural gas stream is flowed through a thin-wall stainless steel tube surrounded by wet hydrogen gas and is heated to thermally decompose the methane. Initially, no fiber growth is observed. However, after several hours, graphite fibers sprout from the inner wall. The fibers grow rapidly in length up to several centimeters and thereafter principally grow radially. In this process, it is believed that the presence of the chromium-bearing steel affects the natural gas pyrolysis in such a manner that the products form the desired fibers.
United States patent application Ser. No. 434,734 filed Oct. 18, 1982, describes an improvement in the process wherein a portion of the stainless steel surface is heavily oxidized prior to growing the fibers. The oxidized surface is positioned in the natural gas downstream from natural stainless steel that has not been heavily oxidized. Pyrolysis initially occurs adjacent the natural surface and the product stream flows over the pretreated surface to grow fibers thereon.
Although prior oxidation significantly increases the yield of fibers on a surface, the treatment only applies to iron-base surfaces and has several disadvantages when applied to the preferred stainless steel because of the high temperatures necessary for oxidation, significantly higher than the temperatures required for fiber growth. The high temperatures are achieved, for example, by a welding torch; but the torch only heats a localized region at a time and is inconvenient for treating relatively large surfaces. The intense heating also distorts the shape of the surface and affects other properties of the steel, including those properties that affect methane pyrolysis to grow the fibers. Thus, an untreated upstream surface is necessary for initial pyrolysis, which may form a large portion of the total surface available for growing fibers. Furthermore, the stainless steel becomes carburized during the fiber growing process, which embrittles the steel so that it tends to crack when reheated to oxidation temperatures. Thus, this oxidation pretreatment is not suitable for rejuvenating a surface once used for growing fibers.
Therefore, it is an object of this invention to provide an improved process for manufacturing graphite fibers by methane pyrolysis adjacent stainless steel or other suitable chromium-iron alloy, which process comprises pretreating a fiber-growing surface to deposit an iron oxide thereon to increase the yield of product fibers, but which pretreatment does not affect the substrate. The iron oxide is deposited at temperatures sufficiently low to avoid substrate oxidation by an evaporation technique suitable for quickly and conveniently preparing a relatively large growing surface. The iron oxide deposit increases the number of product fibers per unit area. Thus, this invention increases the density of product fibers grown on a large area to greatly increase the total yield, thereby improving the efficiency of the growing process and reducing the cost of the fibers. The pretreatment is so successful in growing fibers that no special wet hydrogen contact of the type described in the aforementioned patent is required. In addition, the pretreatment is suitable for depositing iron oxide on surfaces other than stainless steel, including nonferrous surfaces, to grow profuse fibers on surfaces that have not heretofore produced fibers in significant numbers.
It is a more particular object of a preferred embodiment of this invention to pretreat a stainless steel surface to enhance graphite fiber growth thereon by methane pyrolysis without altering the character of the stainless steel to interact with methane during pyrolysis in the manner necessary to produce the desired fibers. The pretreatment is not limited to downstream surfaces, but is applicable for any and all fiber-growing surfaces, thereby maximizing the fiber yield. In addition, the pretreatment is suitable not only for enhancing fiber growth on virgin surfaces, but also for rejuvenating surfaces previously employed for fiber growth. Thus, this invention permits a stainless steel surface to be repeatedly and indefinitely employed for growing batches of fibers. It has also been found that the growing time is reduced for rejuvenated surfaces, thereby further reducing the cost of the fibers.